3d Grease pencil fill
To convert any Mesh object to 3D Grease Pencil object we need Animation Nodes with Extra Nodes(Builds available for Linux, Windows),

(1) First we have to get all the individual polygon (face) vertices using Mesh Object Node and Separate Polygons Node. Separate Polygons Node is needed because all
polygons are joined i.e., they are sharing some vertices,



(2) then convert a polygon vertices to a close (cyclic) grease pencil stroke with fill using GPencil Stroke Output Node.

(3) Because a mesh has many polygons (faces) so we are going to use the Loop Group Node (Loop Input) to convert all polygons to fill grease pencil strokes (cyclic).



(4) Now we have all strokes as output from the Loop Node, with GPencil Object Output Node we can convert them to a grease pencil object.



Happy Noding :)